Stab-type coupling and method

ABSTRACT

A stab-type coupling and method for connecting an end portion of a smooth wall non-metallic pipe to fittings or other pipes. The coupling is defined by a non-metallic coupling body, a generally elongated hollow insert, annular spacer rings, seal rings, a collet ring, and a retaining collet all disposed within the recess of the coupling body. The retaining collet is provided with a tapered external surface which is complementary to an internal frusto-conical surface defined by a back-up ring molded within the coupling body. The coupling is made up as by inserting the end of a smooth wall non-metallic pipe into the coupling body such that the pipe is received about the generally elongated hollow insert and within the retaining collet. The toothed internal surface of the retaining collet is adapted to engage the non-metallic pipe. Attempted withdrawal of the pipe from the coupling body is resisted by the collet toothed surface as well as the bearing engagement of the collet with the back-up ring molded into the coupling body.

This is a continuation of application Ser. No. 899,813, filed Apr. 25,1978.

BACKGROUND OF THE INVENTION

This invention relates to stab-type couplings or couplings in whichassembly of a smooth wall tube to a coupling body is accomplished bypushing or thrusting the tube into the coupling body. Gripping of thetube surface and the resistance to attempted withdrawal is based on the"Chinese finger" principle. A segmented retaining collet cooperates withthe tube such that the greater the pull-out force the more securely thetube becomes seated within the coupling.

Stab-type couplings have been known to the prior art for a considerableperiod of time. Such couplings are relatively easy to use ininstallation and require merely severing the end of the tube at rightangles and chamfering of the tube end to facilitate insertion of thetube into the coupling body. As an example, prior art stab-typecouplings have used both solid and split-type gripper rings, solid orsplit-type gripper washers and various spring-type elements.

One problem encountered by stab-type couplings in the prior art is thecomplexity of design which requires special machining of parts resultingin increased costs. A further problem encountered in the prior art hasbeen the relatively complicated assembly procedure that must bepracticed with certain designs.

With the foregoing in mind, it is an object of the present invention toprovide a stab-type coupling which provides a simple reliable connectionfor on site installation of new pipe and tubing or repair of damagedpipe and tubing without the need for cathodic protection.

BRIEF SUMMARY OF THE INVENTION

Briefly summarized, this invention provides for an improvement instab-type couplings which is utilized for interconnecting the endportion of a smooth wall non-metallic pipe to a fitting or another pipe.The coupling is defined by a non-metallic coupling body having a firstend and an internal recess adapted to receive the end portion of asmooth wall non-metallic pipe. The first end is provided with a moistureseal lip adapted to surround the pipe in a slight interference fit whenthe pipe is inserted into the body. A generally elongated hollowmetallic insert is disposed within the recess of the coupling body. Thesmooth wall non-metallic pipe is adapted to be pushed over the insert asthe pipe is installed in the coupling body. One or more seal rings aredisposed within the recess and are adapted to sealingly engage the pipeand the coupling body to provide sealing within the coupling body. Aretaining collet is disposed within the recess and is provided with atapered external surface which converges in the direction of the firstend of the coupling body. The collet is provided with a toothed internalsurface adapted to engage the smooth wall non-metallic pipe. A generallyfrusto-conical back-up ring is molded into and retained by the couplingbody and is provided with an internal surface generally complementary toan external surface of the collet. The relationship of the collet to theback-up ring is such that the collet is adapted to bear against theback-up ring in the event of an attempted withdrawal of the smooth wallnon-metallic pipe from the coupling body.

A double ended stab-type coupling is provided for interconnecting afirst non-metallic tube or pipe to a second non-metallic tube or pipe.The double ended coupling is defined by a pair of non-metallic couplingbodies which are butt fused together to define a single fluid tightcoupling body into which a pair of smooth wall non-metallic pipes may beinserted.

The invention also contemplates a method of assembly of a double endedstab-type coupling.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete description of the invention will now be made withreference to the accompanying drawings in which:

FIG. 1 is an elevational cross-sectional view of the preferredembodiment of the coupling of this invention showing the interconnectionof a tube to a coupling body;

FIG. 2 is an elevational cross-sectional view of the coupling of thisinvention as provided in a double ended coupling body embodiment;

FIG. 3 is an elevational view of the retaining collet of the coupling ofthis invention;

FIG. 4 is an elevational cross-sectional view of a modification of thesealing structure of the invention;

FIG. 5 is an elevational cross-sectional view of a further modificationof the sealing structure of the invention;

FIG. 6 is an elevational cross-sectional view of a still furthermodification of the sealing structure of the invention; and

FIG. 7 is an elevational cross-sectional view of a modification of thedirt and moisture cover of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will first be made to FIG. 1 wherein there is shown thepreferred embodiment of the stab coupling of this invention. Thestructure of FIG. 1 shows the mode of connection of a smooth wallplastic tube to a coupling body. It should be appreciated that thecoupling body may take several different forms and that the structure ofFIG. 1 depicts only that portion of the coupling body which cooperateswith the tube for purposes of forming an interconnection or jointtherewith.

The coupling of FIG. 1 includes a non-metallic coupling body 12 having afirst end 14 and an internal recess 16 which is adapted to receive theend portion 18 of a smooth wall non-metallic tube or pipe 20.

In the embodiment of FIG. 1 coupling body 12 defines an internalgenerally cylindrical surface 22. A generally frusto-conical metallicback-up ring 24 is molded into coupling body 12 and defines an internalgenerally frusto-conical surface 26 which is adapted to cooperate withretaining collet 28 in a manner to be described more fully below.

Near the first end 14 of coupling body 12 there is defined a generallycylindrical internal surface 30 which is adapted to closely surround theexternal surface of tube 20 when inserted into the coupling body.

Molded integrally with coupling body 12 in the preferred embodiment ofFIG. 1 is a generally outwardly extending flexible flange 32 whichserves to define a dirt and moisture cover to retard or prohibitaltogether the entry of dirt and moisture into the interior of thecoupling body 12. It should be appreciated that the relationship betweencover 32 and tube 20 is such that the cover sealingly engages theexternal surface of tube 20 through a slight interference fit as it isinserted into the coupling body 12.

In the preferred embodiment of FIG. 1 coupling body 12 is injectionmolded from polyethylene or other suitable plastic. Back-up ring 24 isfabricated from type 316 stainless steel and is insert molded into body12.

Disposed within the recess of body 12 and approximately concentric withthe axis thereof is a steel reinforcing stiffener or hollow insert 34. Agenerally cylindrical internal passageway 36 is defined within stiffener34 for the purpose of permitting the passage of fluid therethrough. Theexternal surface 38 of stiffener 34 is received within the interior ofend portion 18 of tube 20. External surface 38 may either be in closesealing engagement with the internal surface of tube 20 or, alternately,may be loosely received therein if desired. While in the preferredembodiment the external surface 38 of insert 34 is generally smooth, itmay be provided with a plurality of teeth 40 (FIG. 4). The purpose andfunction of teeth 40 is to provide for firm retention of the stiffener34 within the tube 20.

At the first end 41 of stiffener 34 there is provided a chamfer 42 inorder to facilitate the entry of the stiffener 34 into tube 20 as thetube 20 is thrust into the coupling body. At the second end 43 ofstiffener 34 there is provided a generally radial outwardly extendingflange 44. The dimension of the outer surface 46 of flange 44 is suchthat the flange is closely received within the cylindrical surface 22 ofcoupling body 12. That is to say, there is a slight clearance providedbetween the outer surface 46 of flange 44 and surface 22. Surface 22thus cooperates with flange 44 in order to center, align and maintainstiffener 34 within coupling body 12 in a position approximatelyconcentric with the axis of the coupling body 12. This relationship ofthe coupling body to flange 44 as just described is particularly helpfulduring assembly of the coupling as will be described below.

Sealing between the tube 20 and the coupling body 12 is provided bymeans of "O" ring seal 48, 49. As will be observed in FIG. 1 seals 48,49 are positioned along the interior cylindrical surface 22 of thecoupling body 12. The relationship of "O" ring seals 48, 49 to tube 20and coupling body 12 is such that when the tube is inserted into thecoupling body there is caused to be achieved a slight compression of the"O" ring seals 48, 49 with the result that sealing is achieved at theexternal surface of tube 20 and at the interior cylindrical surface 22of the coupling body 12.

A generally annular spacer ring 50 is disposed essentially concentricwith the axis of coupling body 12 adjacent flange 44. In the preferredembodiment of FIG. 1 spacer ring 50 is defined by a chamfer 53 and isfabricated from polyethylene material. The spacer 50 is structured to beloosely received about the external surface of tube 20. Spacer ring 50functions to both position "O" ring seals 48, 49 in the manner shown inFIG. 1 and to provide support for flange 44 of stiffener 34 prior to theentry of tube 20 into the coupling body. As will be observed in FIG. 1spacer ring 50 is defined by essentially planar side surfaces 52, 54.Side surface 52 cooperates with flange 44. Surface 54 is adjacentretaining ring 55.

Positioned on either side of "O" ring seals 48, 49 are thrust washers57, 59. These washers function to position "O" ring seals 48, 49 and toseparate the seals from the retaining collet 28 and the retaining ring55. Washer 59 prevents movement of seal 49 into the recesses of thecollet as well as any movement of the seal into the area of thefrusto-conical surface 26. Seal 49 is thus captured or fixed in theposition shown in FIG. 1 so as to be in contact with surface 22.Similarly washer 57 prevents movement of seal 48 into the retaining ring55.

Retaining ring 55 is defined by a plurality of teeth 61 which areinclined in the direction of flange 44. Teeth 61 are dimensioned so asto provide an interference fit with tube 20 when inserted through ring55. The purpose of the ring 55 is to retain the seals 48, 49, thrustwashers 57, 59 and collet 28 in place about tube 20 in the positionshown in FIG. 1. Attempted withdrawal of the tube from the couplingcauses the seals, washers and collet to move as a unit to the right ofFIG. 1 thus enhancing and supplementing the gripping of tube 20 by thecollet and ring 55 as will be described below.

Positioned between washer 59 and first end 14 of the coupling body isretaining collet 28. As best seen in FIGS. 1 and 3 collet 28 is providedwith a tapered external surface 56 which converges in the direction ofthe first end 14 of the coupling body 12. Tapered surface 56 isgenerally complementary to frusto-conical surface 26 defined by back-upring 24 and is adapted to bear against surface 26 during such time as apull-out force is exerted on tube 20. The interior of collet 28 isdefined by a toothed surface generally designated 58. As will be evidentfrom an inspection of FIG. 1 toothed surface 58 is defined by aplurality of inclined ramp surfaces 60 which diverge in the direction ofthe first end 14 of the coupling body 12. Ramp surfaces 60 areperiodically interrupted by radial surfaces 62 which cooperate with rampsurfaces 60 to define edges 64 which provide for biting engagement intothe external surface of tube 20. The orientation of the plurality ofedges 64 and teeth 61, as viewed in FIG. 1, is away from first end 14 ofcoupling body 12. As a result an attempted withdrawal of tube 20 fromcoupling body 12 as by moving tube 20 to the right of FIG. 1 will causethe respective edges 64 and teeth 61 to bite into the external surfaceof tube 20.

While in the preferred embodiment as shown in FIG. 1 surfaces 62 areessentially radially oriented relative to the axis of the coupling body12 it should be appreciated that in alternate embodiments surfaces 62may be inclined relative to the axis of coupling body 12.

As will be observed in FIG. 3 collet 28 is provided with a plurality ofgenerally longitudinally extending recesses 68, 70. The recesses arestaggered such that alternate recesses 68 will open toward the secondend 72 of collet 28 while corresponding alternate recesses 70 opentoward first end 74. As will be observed from FIG. 1 the respectiverecesses 68, 70 extend through the entire radial thickness of the collet28. The recesses do not extend through the entire collet in alongitudinal direction, however, since there are provided uninterruptedareas 76, 78 at the respective ends of the recesses 68, 70 as will beobserved in FIG. 3.

The purpose and function of the recesses 68, 70 of collet 28 is topermit contraction of the collet at such time as a pull-out force isexerted on tube 20 and the collet is brought into abutting engagementwith back-up ring 24 of the coupling body 12. As will be appreciatedfrom a study of FIG. 1 the application of a pull-out force to tube 20causes collet 28 to be cammed into back-up ring 24. As a result thecollet is caused to be uniformly contracted circumferentially. Recesses68, 70 provide an advantageous structure for absorbing contractions ofthe collet without cracking or distortion of the collet material.

Similarly, the alternately spaced longitudinal recesses 68, 70 permitthe collet to expand radially outwardly. In the preferred embodiment thecollet is manufactured such that the internal dimension of the toothedsurface 58 is slightly less than the outer diameter of the tube. As aresult there is established an initial interference fit between the tubeand the collet whereby the collet is expanded slightly outwardly as thetube is inserted into the coupling body. As a consequence of preloadingthe collet there is achieved greater initial holding of the colletrelative to the tube.

In the preferred embodiment retaining collet 26 is molded from an acetalcopolymer. Other materials such as brass, stainless steel, plastics andnon-corrosive metallic materials should be considered within the spiritof this invention. Similarly the use of reinforced plastics may bedesirable.

As will be appreciated from a discussion of the invention thus far allof the elements of the coupling with the exception of the back-up ring24 and stiffener 34 are fabricated from non-metallic materials. In thepreferred embodiment the back-up ring is fabricated from stainless steelwhich renders it essentially impervious to corrosion. Stiffener 34 maybe fabricated from steel or stainless steel. As a consequence thecoupling of this invention is non-corrosive in nature and is suitablefor use with plastic pipe. It should be appreciated, however, thatalternate materials for the several components of the coupling may beselected including metals and other plastics. Similarly while thepreferred embodiment of the invention has been described in connectionwith plastic tube or pipe it should be appreciated that metallic tubesor pipes may be used as well.

Thus far the invention as embodied in FIG. 1 has been described withreference to a coupling body 12 having a first end 14. There has been nodescription of the characteristics of the second end of the couplingbody which is generally designated 80 in FIG. 1. It should be understoodin this regard that the coupling body may take alternate forms insofaras the configuration of the second end. For example, the second end 80could be provided with external male threads defining a male connectorend. Second end 80 could be provided with female threads rendering thecoupling suitable for use as a female connector. Alternately thecoupling could be used as a reducing fitting, an L fitting, a T fitting,a transition fitting or other type of fitting known to the industry.Alternate connecting means could be employed at the second end 80 of thecoupling body including fusion welding connecting means, ferrule-typeconnecting means or other connecting means well known to those skilledin the art.

FIG. 2 illustrates a double ended modification of applicant's inventionwherein a pair of coupling structures of the type shown in FIG. 1 arecaused to be joined in a "back-to-back" relationship for purposes ofmaking a pipe-to-pipe or tube-to-tube connection. For ease ofdescription many of the components of the coupling of this inventionhave been omitted from FIG. 2 since the principal attention of FIG. 2 isthe relationship of duplicate coupling segments which have been joinedtogether.

In FIG. 2 coupling body 12 is shown joined to a similar coupling body12' by means of a butt heat fused joint 82. Joint 82 is formed by firstcausing to be heated opposed planar surfaces of the respective couplingbodies 12, 12'. Once heated to appropriate fusing temperature thecoupling bodies are caused to be drawn together under pressure resultingin the formation of a fused joint 82. When the joint is cooled there isdefined a raised shoulder 84 extending circumferentially about both theinterior and exterior of the coupling bodies 12, 12'.

Prior to fusing of the respective coupling bodies 12, 12' of FIG. 2together each of the respective coupling bodies are caused to beassembled as follows. The retaining collet 28 is first caused to beinserted into the coupling body 12 by passing the collet through thesecond end thereof. Thereafter the thrust washer 59 and "O" ring seals48, 49 are passed through the second end of the coupling body intoposition just behind the collet. The thrust washer 57, ring 55 andspacer 50 are then positioned adjacent the "O" ring seal 48 followed byplacement of the reinforcing stiffener 34. With the elements assembledin each of the respective coupling bodies 12, 12', the bodies are fusedtogether in the manner described above. Once the coupling bodies arejoined together the several components of the coupling are positionedwithin the coupling body in a manner to receive the respective endportions of tubes or pipes. Each tube is caused to be joined to thecoupling by thrusting or inserting the free end of the tube into thecoupling body in a manner such that the stiffener 34 is received withinthe tube while, at the same time, the tube is caused to be passed withinthe collet 28, retaining ring 55, thrust washers 57, 59, "O" ring seals48, 49 and spacer ring 50. In installation the tube is insertedcompletely into the coupling body until the free end 86 thereof iscaused to bottom against flange 44.

Prior to insertion of the tube into the coupling body a right angle cutis made in the tube in order to define a free end 86 (FIG. 1) which isessentially radial in orientation relative to the axis of the tube. Forease of installation the tube is then chamfered at 88 in order tofacilitate the insertion of the tube through the collet, the thrustwashers, retaining ring, seal rings and the spacer. Chamfering may beaccomplished using any conventional bevel tool. At the time the tube ischamfered provision may be made for causing a mark to be placed at theexterior surface of the tube in order to insure that the tube isinserted into the coupling to a proper depth when the coupling is madeup.

It should be appreciated that with the exception of the insertion of thefree end of the tube into the coupling body all assembly operationsconcerning the coupling are carried out in the coupling manufacturer'splant. That is to say, the coupling body, collet, thrust washers,retaining ring, seal rings, spacer ring and stiffener are initiallyassembled in the manner shown in FIG. 1 in the manufacturer's plant andshipped as a self-contained unit ready to receive the free end of a tubeor pipe in the field. When received in the field, therefore, thecoupling unit requires no further assembly other than to cut and chamfera section of pipe or tubing to be joined and, thereafter, insert thepipe or tubing into the coupling. There is no fusion joint made in thefield nor is there any necessity to assemble the components in thefield. This eliminates concern of incorrect assembly oftentimes raisedin connection with prior art structures wherein the installer mustassemble all the components of the coupling in the field underconditions less than optimum.

Whether the coupling body 12 takes the form of a male connector, femaleconnector, double ended embodiment as shown in FIG. 2 or any alternateembodiment, the coupling body is self-contained and ready to receive thefree end of a section of pipe or tube when shipped to the job site.

As has previously been noted, the radial dimension of flange 44 is suchthat the flange is closely received within cylindrical surface 22 ofcoupling body 12. As a consequence the stiffener 34 is caused to becaptured within the coupling body in a position approximately concentricwith the axis thereof so that there is defined a generally annularrecess between the stiffener and the collet, thrust washers, retainingring, seal rings and spacer ring elements. Such recess is, of course,for the purpose of receiving the free end of a length of pipe or tube.

In FIG. 2 it should be observed that the raised shoulder 84 defines, atthe interior of the coupling bodies 12, 12' a back-up surface whichcooperates with surface 52 of spacer ring 50 in order to trap flange 44therebetween. Depending on the dimensional relationship of the severalelements of the coupling it may be considered desirable to provide formore or less space between flange 44 and raised shoulder 84 of buttjoint 82 than that shown in FIG. 2. Alternate spacing relationshipsshould be considered within the scope and spirit of this invention.Shoulder 84 also functions to inhibit interference of one side of thedouble ended coupling with the other. That is to say, shoulder 84inhibits movement of flange 44 to the left (or right as the case may be)so as not to interfere with the opposed flange.

In the preferred embodiment the coupling body 12 is provided with aninsert molded stainless steel back-up ring 24 which defines a rigidcamming surface into which the collet 28 is caused to bear in the eventof the application of a pull-out force to the tube 20. Alternateembodiments should be considered within the scope and spirit of thisinvention including structure wherein the stainless steel back-up ring24 is eliminated entirely from the coupling body and, in place thereof,a frusto-conical surface 26 is defined by the coupling body itself.

A first modification of the seal structure of the invention is shown inFIG. 4. A pair of O-rings 90, 92 sealingly engage the external surfaceof tube 98 and the internal surface 94 of the coupling body 96. Spacer100 is positioned between O-rings 90, 92. Spacer 102 is positionedbetween O-ring 90 and flange 104 of insert 106. Shim spacer ring 108 ispositioned between collet 110 and O-ring 92.

A second modification of the seal structure of the invention is shown inFIG. 5. A single O-ring 112 sealingly engages the external surface oftube 116 and the internal surface 118 of the coupling body 120. Spacer122 is positioned between O-ring seal 112 and flange 124 of insert 126.Shim spacer ring 128 is positioned between collet 130 and O-ring 112. Inthe modification of FIG. 5 the O-ring seal 112 makes sealing contactwith the external surface of tube 116 and surface 118 of body 120.

A third modification of the seal structure of the invention is shown inFIG. 6. An "H" type seal 134 sealingly engages the external surface oftube 136 at two spaced apart locations thereon. Similarly seal 134sealingly engages surface 138 of coupling body 140 at two spaced apartlocations thereon. Spacer 142 is positioned between seal 134 and flange144 of insert 146. Shim spacer 148 is positioned between collet 150 andseal 134.

A modification of the dirt and moisture cover of the invention is shownin FIG. 7. An elastomeric or plastic sleeve 152 is secured to thecoupling body 154 and sealingly engages the external surface of tube156. Sleeve 152 may be dip molded to the coupling body 154 and extendedinto place as shown in FIG. 7 after insertion of tube 154 into thecoupling body. In addition an O-ring seal 158 is shown disposed inflange 160 of body 154. O-ring seal 158 sealingly engages tube 156 atthe external surface thereof.

The method of this invention for the assembly of a double endedstab-type coupling for connecting a first smooth wall non-metallic pipeto a second smooth wall nonmetallic pipe comprises the steps of:

(a) providing a first non-metallic coupling body having a first end anda second end and an internal recess adapted to receive the end portionof the first smooth wall non-metallic pipe, said internal recessdefining a generally frusto-conical surface;

(b) providing a second non-metallic coupling body having a first end anda second end and an internal recess adapted to receive the end portionof the second smooth wall non-metallic pipe, said internal recessdefining a generally frusto-conical surface;

(c) providing a first and second elongated hollow insert, each of saidinserts having a radial flange extending outwardly therefrom;

(d) providing a first and second seal ring;

(e) providing a first and second retaining collet, each of said colletshaving a tapered external surface which converges toward one end thereofand a toothed internal surface;

(f) providing a first and second generally annular spacer ring;

(g) providing at least one first and second generally annular thrustwasher;

(h) inserting a respective retaining collet, thrust washer, seal ring,spacer ring, and hollow insert into a respective coupling body such thatthe internal surface of each respective coupling body is generallycomplementary to the external surface of each respective retainingcollet and the radial flange of each respective hollow insert isgenerally disposed adjacent the second end of each respective couplingbody;

(i) fusing the respective coupling bodies together at the second endsthereof.

ADVANTAGES OF THE INVENTION

Applicant's improved coupling and method provide numerous advantages.Applicant's containment mechanism provides that the plastic tubing isfully contained and retained between metallic members which cooperatewith or are contained within the non-metallic coupling body.

In the design of non-metallic tube fittings the creep phenomenon ofcertain plastics can cause a loss of the gripping and sealing functionof various elements of the fitting. Applicant, on the other hand, hasprovided in the fitting and method of this invention that the plastictubing be contained between metallic members which are not subject tocreep. Referring to FIG. 1 it can be appreciated that end portion 18 oftube 20 is trapped between the metallic stiffener 34 and the back-upring 24. As a consequence the back-up ring and stiffener define aconstant dimension which receives the plastic tubing there being novariation in the dimension as a consequence of creep.

A further advantage of applicant's invention resides in the fact thatthe steel reinforcing stiffener is designed as an integral part of thecoupling as initially assembled at the manufacturer's plant. Thebuilt-in feature of the stiffener eliminates the necessity of requiringthe installer in the field to place the stiffener in the tube end beforeinsertion of the tube into the coupling body. In many cases it has beenfound that the installer will neglect to install the stiffener in thefield. Unfortunately, the absence of a stiffener is not capable of beingchecked by quality control personnel since, once assembled, thestiffener is no longer in view. Accordingly, in providing for a built-instiffener applicant has eliminated the possibility of an installerinadvertently failing to insert a stiffener at the time of installation.

A further advantage of applicant's stiffener is that it reinforces thetube end against collapse as a result of loading imparted to the tubeduring pull out.

In addition to defining a stable interior dimension applicant'sstainless steel back-up ring 24 provides increased rigidity against pullout of the collet and tube from the coupling body.

Testing of applicant's coupling has shown that the strength of thefitting significantly exceeds that of the polyethylene or polybutylenepipe or tube used with the fitting. That is to say, in pull-out testsconducted with respect to applicant's device the pipe or tube will failin tension without pull out being observed from the coupling body.

Applicant's coupling assures quick, reliable on site repairs andinstallation of pipe and tubing in the field. The "Chinese finger" stabprinciple provides for increased resistance to pull out forces as afunction of increased magnitude of such forces. As the magnitude of thepull out force on the tube increases the collet exerts a greatergripping force on the surface of the tube causing tube and collet tobecome more seated within the coupling.

What is claimed is:
 1. A method of assembly of a double ended stab-typecoupling for connecting a first smooth wall non-metallic pipe to asecond smooth wall non-metallic pipe, said method comprising the stepsof:(a) providing a first non-metallic coupling body having a first endand a second end and an internal recess adapted to receive the endportion of the first smooth wall non-metallic pipe, said internal recessdefining a generally frusto-conical surface; (b) providing a secondnon-metallic coupling body having a first end and a second end and aninternal recess adapted to receive the end portion of the second smoothwall non-metallic pipe, said internal recess defining a generallyfrusto-conical surface; (c) providing a first and second elongatedhollow insert, each of said inserts having a radial flange extendingoutwardly therefrom; (d) providing a first and second seal ring; (e)providing a first and second retaining collet, each of said colletshaving a tapered external surface which converges toward one end thereofand a toothed internal surface; (f) providing a first and secondgenerally annular spacer ring; (g) inserting a respective retainingcollet, seal ring, spacer ring, and hollow insert into a respectivecoupling body such that the internal surface of each respective couplingbody is generally complementary to the external surface of eachrespective retaining collet and the radial flange of each respectivehollow insert is generally disposed adjacent the second end of eachrespective coupling body; (h) fusing the respective coupling bodiestogether at the second ends thereof.
 2. A method of assembly of a doubleended stab-type coupling for connecting a first smooth wall non-metallicpipe to a second smooth wall non-metallic pipe, said method comprisingthe steps of:(a) providing a first non-metallic coupling body having afirst end and a second end and an internal recess adapted to receive theend portion of the first smooth wall non-metallic pipe; (b) providing asecond non-metallic coupling body having a first end and a second endand an internal recess adapted to receive the end portion of the secondsmooth wall non-metallic pipe; (c) providing hollow insert means; (d)providing a first and second seal ring; (e) providing a first and secondretaining ring, each of said retaining rings having locking teeth; (f)providing a first and second generally annular spacer ring; (g)inserting a respective seal ring, retaining ring, and spacer ring into arespective coupling body; (h) inserting said hollow insert means intosaid coupling bodies; (i) fusing the respective coupling bodies togetherat the second ends thereof.